Trace and low concentration CO2 removal methods and apparatus utilizing metal organic frameworks

ABSTRACT

Embodiments of the present disclosure describe a device for removing CO 2  comprising a gas flow inlet, a housing including a SIFSIX-3-Cu metal-organic framework (MOF) composition for sorbing and/or desorbing CO 2 , and a gas flow outlet. Embodiments of the present disclosure describe an anesthetic system comprising one or more regeneratable cartridges for sorbing and/or desorbing CO 2 , wherein each of the one or more regeneratable cartridge includes a metal-organic framework composition, wherein at least one of the regeneratable cartridges includes a SIFSIX-3-Cu MOF. Embodiments of the present disclosure describe an alkaline fuel cell comprising a catalyst layer including a SIFSIX-3-Cu MOF composition for sorbing and/or desorbing CO 2 .

BACKGROUND

Direct air capture can mitigate the increasing CO₂ emissions associatedwith the carbon polluting sources. Efficient and cost-effective removalof trace CO₂ is important in various key industrial applicationspertaining to energy, environment and health. From an industryprospective, the removal of trace CO₂ from air is a growing area ofresearch and development due to its substantial importance forpre-purification of air and particularly when atmospheric air is usedduring the separation of nitrogen and oxygen.

The amount of CO₂ in the atmosphere continues to rise rather rapidly dueto unparalleled cumulative CO₂ emissions, provoking the undesirablegreenhouse gas effect. Certainly, it is becoming critical to developeconomical and practical pathways to reduce CO₂ emissions. Appropriatelyprospective routes to address this enduring challenge have beenconsidered: (i) CO₂ emission reduction from post-combustion stationaryand mobile sources where CO₂ concentration is in the range of 10-15% and(ii) CO₂ removal from air, called direct air capture (DAC), which isanother alternative option to reduce greenhouse gases emissions in auniform way globally. Although DAC is relatively more challenging thanpost-combustion capture, it is recognized that it might be practical,provided that suitable adsorbent combining optimum uptake, kinetics,energetics and CO₂ selectivity is available at trace CO₂ concentrations.

In an example, prior to air separation using cryogenic distillation orpressure swing adsorption (PSA), air must be CO₂ free to avoid (i)blockage of heat-exchange equipment as a result of frozen CO₂ during theliquefaction process and (ii) adsorbents (e.g., zeolites) contaminationused for oxygen production by pressure swing adsorption (PSA).

Equally important, alkaline fuel cells (AFCs) require a CO₂ freefeedstock of oxygen and hydrogen gases as it is widely recognized thattrace amounts of CO₂ (i.e. 300 ppm) degrade the electrolyte in AFCs.Furthermore, efficient removal of CO₂ at low concentrations is alsovital for the proper operation of breathing systems in confined spacessuch as submarines and aerospace shuttles.

Efficient CO₂ removal and resupply of fresh air is also critical inmining and rescue missions, diving, and most importantly in medicalapplications such as anaesthesia machines. The use of anaesthesiamachine is still a growing clinical trend worldwide, driven by the needto reduce cost and improve patient care via the use of efficient CO₂sorbents. A CO₂ removal feature in anaesthesia machine is particularlyimportant in semi-closed or closed rebreathing systems, as therebreathing fraction is at least 50% of the exhaled gas volume, directedback to the patient after proper CO₂ removal in the next exhalation.Currently, common sorbents for this application are non-recyclable, andgenerate large amounts of unwanted medical waste.

There is a pressing need to develop novel porous materials that canadequately address the growing interest to low CO₂ concentration removalapplications. Only a few materials were reported to adsorb efficientlytraces of CO₂, particularly with regards to DAC using a variety of aminesupported materials (e.g. porous silica). However, these materialscontain primary amines which require high energy for regeneration, suchas about 80-120 kJ/mol, in part due to the materials' chemicaladsorption mechanisms. Additionally, amine grafting is conducted in astep separate from the platform material synthesis, thus addingadditional cost and time to manufacturing.

Modular and tunable porous materials, namely metal-organic frameworks(MOFs), can be used to tackle this ongoing challenge. Recently, MOFswere intensively investigated for intermediate and high CO₂concentration removal applications such as post-combustion,pre-combustion capture, natural gas and biogas upgrading. Nevertheless,the potential of MOFs to remove traces and low CO₂ concentration fromgas streams was rarely considered. The main reason for this lack ofstudies is that most MOFs reported so far, with or without unsaturatedmetal sites (UMC) or/and functionalized ligands, exhibit relatively lowCO₂ selectivity and uptake particularly at relatively low CO₂ partialpressure. To overcome this downfall, various research groups haveadopted the amine grafting chemistry and the acquired knowledge fromamine-supported silica, as a prospective pathway to enhance the CO₂adsorption energetics and uptake in MOFs and covalent organic frameworks(COFs). Markedly, the few reported strategies targeting air captureusing MOFs are centred on the aptitude of grafted amines to form astrong chemical bond (at least 70 kJ·mol⁻¹) with CO₂, affording highaffinity toward CO₂ and therefore high CO₂ selectivity. Particularly,ethylenediamine (ED) grafting on Mg-MOF-74 supports have been studiedfor CO₂ adsorption from ultra-dilute gas streams such as ambient air.Similarly, N,N-dimethylethylenediamine grafting for DAC using anexpanded isostructure of Mg-MOF-74 has also been studied. All suchmaterials suffer from the drawbacks of amine grafted materials asdiscussed above.

SUMMARY

In general, this disclosure describes techniques for removing CO₂ fromfluids using SIFSIX-n-M MOFs, wherein n is at least two and M is ametal. In some embodiments, the metal is zinc or copper. Embodimentsinclude devices comprising SIFSIX-n-M MOFs for removing CO₂ from fluids.In particular, embodiments relate to devices and methods utilizingSIFSIX-n-M MOFs for removing CO₂ from fluids, wherein CO₂ concentrationis trace. Methods utilizing SIFSIX-n-M MOFs for removing CO₂ from fluidscan occur in confined spaces. SIFSIX-n-M MOFs can comprise bidentateorganic ligands. In a specific embodiment, SIFSIX-n-M MOFs comprisepyrazine or dipryidilacetylene ligands.

Embodiments of the present disclosure describe a device for removing CO₂comprising a gas flow inlet, a housing including a SIFSIX-3-Cumetal-organic framework (MOF) composition for sorbing and/or desorbingCO₂, and a gas flow outlet. Embodiments of the present disclosuredescribe an anesthetic system comprising one or more regeneratablecartridges for sorbing and/or desorbing CO₂, wherein each of the one ormore regeneratable cartridge includes a metal-organic frameworkcomposition, wherein at least one of the regeneratable cartridgesincludes a SIFSIX-3-Cu MOF. Embodiments of the present disclosuredescribe an alkaline fuel cell comprising a catalyst layer including aSIFSIX-3-Cu MOF composition for sorbing and/or desorbing CO₂.

The details of one or more examples are set forth in the accompanyingdrawings and the description below. Other features, objects, andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate non-limiting example embodiments ofthe invention.

FIG. 1A illustrates a SIFSIX-n-M MOF coordinated by pyrazine ligands,according to one or more embodiments.

FIG. 1B illustrates a perspective view of a SIFSIX-n-M MOF coordinatedby dipryidilacetylene ligands, according to one or more embodiments.

FIGS. 1C-D illustrate block flow diagrams of a methods for removing CO₂from a fluid, according to one or more embodiments.

FIG. 2A illustrates CO₂ adsorption isotherms at variable temperaturesfor a SIFSIX-3-Cu MOF, according to one or more embodiments.

FIG. 2B illustrates a pore size distribution for a SIFSIX-3-Cu MOF,according to one or more embodiments.

FIG. 3A illustrates CO₂ volumetric uptake for various SIFSIX metalorganic frameworks, according to one or more embodiments.

FIG. 3B illustrates isosteric heats of adsorption at low coverage forSIFSIX-3-Cu, SIFSIX-3-Zn and SIFSIX-2-Cu—I metal organic frameworks,according to one or more embodiments.

FIG. 4A illustrates a column breakthrough test of CO₂/N₂:1000 ppm/99.9%for SIFSIX-3-Cu and SIFSIX-3-Zn metal organic frameworks in dryconditions, according to one or more embodiments.

FIG. 4B illustrates a column breakthrough test of CO₂/N₂:1000 ppm/99.9%for a SIFSIX-3-Cu metal organic framework in dry conditions as well asat 74% relative humidity (RH), according to one or more embodiments.

FIG. 5 illustrates a schematic view of a device for removing CO₂ from agas, according to one or more embodiments.

DETAILED DESCRIPTION

The present invention is described with reference to the attachedfigures, wherein like reference numerals are used throughout the figuresto designate similar or equivalent elements. The figures are not drawnto scale and they are provided merely to illustrate the invention.Several aspects of the invention are described below with reference toexample applications for illustration. It should be understood thatnumerous specific details, relationships, and methods are set forth toprovide an understanding of the invention. One skilled in the relevantart, however, will readily recognize that the invention can be practicedwithout one or more of the specific details or with other methods. Inother instances, well-known structures or operations are not shown indetail to avoid obscuring the invention. The present invention is notlimited by the illustrated ordering of acts or events, as some acts mayoccur in different orders and/or concurrently with other acts or events.Furthermore, not all illustrated acts or events are required toimplement a methodology in accordance with the present invention.

The present disclosure provides SIFSIX-n-M MOFs for use in trace and lowCO₂ concentration removal, and CO₂ sequestration in confined spaces. Aparticular advantage of SIFSIX-n-M MOFs is high CO₂ removal efficiencyat very low CO₂ partial pressure without any post-functionalization(e.g., amine functionalization), thereby eliminating costly andinefficient processing steps as necessary with amine functionalizedMOFs. A further advantage is the ability to regenerate and reuseSIFSIX-n-M MOFs after CO₂ capture. Generally, energy for regeneratingSIFSIX-n-M MOFs as provided herein is only about 45-55 kJ/mol, ascompared to 80-120 kJ/mol for amine functionalized materials.

As used herein, “trace” refers to species concentrations which are lessthan about 10%, less than about 7.5%, less than about 5%, less thanabout 2.5%, or less than about 1% of a system. “Trace” can additionallyor alternatively refer to a species having a partial pressure belowabout 125 mbar, below about 100 mbar, below about 75 mbar, below about50 mbar, or below about 25 mbar. For example, CO₂ concentration can bedeemed “trace” when the partial pressure of CO₂ in a system is less than50 mbar. An example of a system having a trace CO₂ concentration is asystem having at least 95% N₂. As used herein, “confined spaces” referto areas which have limited or no supply of fresh air. Examples ofconfined spaces include aeronautical vessels such as an airplane orspace, submarines, and industrial vessels such as those with smallhatched openings.

Metal organic frameworks (MOFs) are a versatile and promising class ofcrystalline solid state materials which allow porosity and functionalityto be tailored towards various applications. For example, MOF materialsexhibit exceptionally high specific surface area, in addition to tunablepore size and functionality (e.g., CO₂ selectivity, and H₂O tolerance),which make them suitable for many applications including gas storage,gas separation, catalysis, drug delivery, light-emitting devices, andsensing.

Generally, MOFs comprise a network of nodes and ligands, wherein a nodehas a connectivity capability at three or more functional sites, and aligand has a connectivity capability at two functional sites each ofwhich connect to a node. Nodes are typically metal ions or metalcontaining clusters, and, in some instances, ligands with nodeconnectivity capability at three or more functional sites can also becharacterized as nodes. In some instances, ligands can include twofunctional sites capable of each connecting to a node, and optionallyone or more additional functional sites which do not connect to nodeswithin a particular framework. In some embodiments, ligands can bepoly-functional, or polytopic, and comprise two or more functional sitescapable of each connecting to a node. In some embodiments, polytopicligands can be heteropolytopic, wherein at least two of the two or morefunctional sites are different.

A MOF can comprise a metal-based node and an organic ligand whichextrapolate to form a coordination network. Such coordination networkshave advantageous crystalline and porous characteristics affectingstructural integrity and interaction with foreign species (e.g., gases).The particular combination of nodes and ligands within a framework willdictate the framework topology and functionality. Through ligandmodification or functionalization, the environment in the internal porescan be modified to suit specific applications.

A MOF can be represented by the formula [(node)a(ligand)b(solvent)c]n,wherein n represents the number of molecular building blocks. Solventrepresents a guest molecule occupying pores within the MOF, for exampleas a result of MOF synthesis, and can be evacuated after synthesis toprovide a MOF with unoccupied pores. Accordingly, the value of c canvary down to zero, without changing the definitional framework of theMOF. Therefore, in many instances, MOFs as provided herein can bedefined as [(node)_(a)(ligand)_(b)]_(n), without reference to a solventor guest molecule component.

An example of a class of MOFs is SIFSIX-n-M, wherein n is at least two,and M can comprise Cu, Zn, Co, Mn, Mo, Cr, Fe, Ca, Ba, Cs, Pb, Pt, Pd,Ru, Rh, and Cd. The SIFSIX-n-M MOF class is isoreticular across itsmetal analogues (i.e., each M analogue has the same framework topology)and is characterized by periodically arrayed hexafluorosilicate (SIFSIX)octahedral pillars. SIFSIX-n-M MOFs have many desirable characteristics,including tunable pore sizes, which lend the various analogues well to anumber of industrial applications. FIGS. 1A-B show examples ofSIFSIX-n-MOF analogues.

FIG. 1A illustrates a SIFSIX-n-M MOF coordinated by pyrazine ligands.Specific analogues of this MOF include SIFSIX-3-Cu and SIFSIX-3-Zn,among others. Such SIFSIX-3-M analogues are iso-structural, based onpyrazine/M(II) 2-D periodic 4⁴ square grids pillared by (SiF₆)²⁻ anions.SIFSIX-3-Zn MOFs comprising pyrazine ligands can have average pore sizesof about 3.84 Å and BET apparent surface areas of about 250 m²·g⁻¹(determined from the CO₂ adsorption isotherm at 298K). SIFSIX-3-Cu MOFscomprising pyrazine ligands can have average pore sizes of about 3.50 Å(NLDFT) and BET and Langmuir apparent surface areas of ca. 300 m²·g⁻¹(determined from the CO₂ adsorption isotherm at 298K). FIG. 1Billustrates a SIFSIX-n-M MOF coordinated by dipryidilacetylene (DPA)ligands. A specific analogue of this MOF is SIFSIX-2-Cu-i, among others.SIFSIX-2-Cu—I MOFs comprising DPA ligands can have average pores size of5.15 Å and BET apparent surface areas (determined by N₂ adsorption) ofabout 735 m²·g⁻¹. In FIGS. 1A-B, guest molecules have been omitted forclarity.

SIFSIX-n-M MOFs can be coordinated by a variety of organic ligands. Insome embodiments, the ligand can be any bidentate (i.e., bi-functional)N-donor linkers based on monocyclic or polycyclic group (aromatic ornot). In some embodiments, a ligand can comprise a polydentate, orpoly-functional ligand, such as a bi-functional ligand, a tri-functionalligand, or ligands with four or more functional sites. In someembodiments, a ligand can comprise an N-donor linker. In someembodiments a ligand can comprise a poly-functional ligand. In someembodiments, a ligand can comprise a plurality of N-donor functionalgroups. Pyrazine is an example of a ligand with two N-donor functionalgroups. In some embodiments, a ligand can comprise a monocyclic orpolycyclic group structure, wherein the cyclic groups can be aromatic ornonaromatic. In some embodiments, a ligand can comprise anitrogen-containing monocyclic or polycyclic group structure. In someembodiments, a ligand can comprise a nitrogen-containing heterocyclicligand, including pyridine, 4,4′-Bipyridin, pyrazine, pyrimidine,pyridazine, triazine, thiazole, oxazole, pyrrole, imidazole, pyrazole,triazole, oxadiazole, thiadiazole, quinoline, benzoxazole,benzimidazole, 1,4-Diazabicyclo [2.2.2]octane (DABCO),1,2-bis(4-pyridyl)acetylene (dpa), and tautomers thereof.

The SIFSIX-n-M MOFs presented herein provide uniformly distributed andnon-reactive CO₂ adsorption energetics and remarkable CO₂ adsorptionproperties, uptake and selectivity in highly diluted gas streams. Suchperformance is currently unachievable with other class of porousmaterials. In particular, SIFSIX-n-M MOFs are suitable for trace CO₂removal applications, due to their strong CO₂ adsorption sites. SIFSIXMOF materials exhibit very high (non-reactive) CO₂ energetics, but fullyreversible physical driven adsorption-desorption operations at very mildconditions. The ideal combination of contracted pore size and the highcharge density also provide unprecedented CO₂ uptake and selectivityover H₂, CH₄ and N₂ at very low partial pressures.

SIFSIX-n-M MOFs are suitable for post-combustion capture (at CO₂ partialpressures of about 100 mbar), but also excellent features suitable fornatural and biogas upgrading as well as pre-combustion capture (high CO₂concentration and high pressure).

FIG. 1C illustrates a block flow diagram of a method 100 for removingCO₂ from a fluid via a SIFSIX-n-M MOF. Method 100 includes contacting102 one or more SIFSIX-n-M MOF compositions with a fluid and sorbing 104CO₂ from the fluid with the one or more SIFSIX-n-M MOF compositions. Inparticular, method 100 can include contacting 102 one or moreSIFSIX-n-Cu and SIFSIX-n-Zn MOF compositions with a fluid and sorbing104 CO₂ from the fluid with the one or more SIFSIX-n-Cu and SIFSIX-n-ZnMOF compositions. In particular, method 100 can include contacting 102one or more SIFSIX-n-M MOF compositions with a fluid and sorbing 104 CO₂from the fluid with the one or more SIFSIX-n-M MOF compositions, whereinM can comprise Cu, Zn, Co, Mn, Mo, Cr, Fe, Ca, Ba, Cs, Pb, Pt, Pd, Ru,Rh, and Cd. SIFSIX-n-M MOFs can comprise bidentate organic ligands.SIFSIX-n-M MOFs can comprise bidentate N-donor organic ligands. Thebidentate N-donor organic ligands can comprise a cyclic moiety. Thebidentate N-donor organic ligands can include monocyclic or polycyclicmoieties. Monocyclic and polycyclic bidentate N-donor organic ligandscan be aromatic and non-aromatic. SIFSIX-n-M MOFs can comprise pyrazineor DPA ligands.

Contacting 102 can include mixing, bringing in close proximity,chemically contacting, physically contacting or combinations thereof.Fluids can include general liquids and gases which include CO₂. Inparticular, fluids can include general liquids and gases which includetrace amounts of CO₂. In some embodiments, fluids include industrialprocess fluids. In some embodiments, fluids include breathing air.Breathing can include any air which is inhaled by a living organism.Breathing air can include air in a confined space. Breathing air caninclude air provided by a breathing device such as a medical device anda SCUBA tank. Fluids can include one or more of water, N₂, O₂, and H₂.In a specific embodiment, fluids can include CO₂ and one or more ofwater, N₂, O₂, and H₂.

In one embodiment, sorbing 104 comprises absorbing. In one embodiment,sorbing 104 comprises adsorbing. In one embodiment, sorbing 104comprises a combination of adsorbing and absorbing. Sorbing 104 caninclude selective sorption of CO₂ over other species present within thefluid. For example, sorbing 104 can include selectively sorbing CO₂ overone or more of water, N₂, O₂, and H₂. The SIFSIX-n-M MOF compositionscan sorb about 1% to about 99.9%, about 1% to about 90%, about 1% toabout 50% or about 1% to about 30% of one or more compounds in a fluid.Sorbing 104 can include reducing the CO₂ concentration in a fluid toless than about 1%, less than about 0.5%, less than about 0.1%, or lessthan about 0.01%.

Sorbing 104 can occur at ambient temperature, at an elevatedtemperature, at a cooled temperature, or over a temperature range. Inone embodiment, a temperature can be selectively changed to manipulatesorption and/or desorption of different compounds. Sorbing 104 can occurat ambient pressure, at an elevated pressure, at a cooled pressure, orover a pressure range. In one embodiment, pressure can be selectivelychanged to manipulate sorption and/or desorption of different compounds.In addition to or in the alternative to, a concentration of one or moreSIFSIX-n-M MOF compositions can be varied to affect a rate and/ormagnitude of sorbing 104. One or more of temperature, pressure andSIFSIX-n-M MOF concentration can be regulated to produce a simultaneoussorption of compounds, or a subsequent, step-wise sorption (i.e., inseries) of compounds from a fluid. In series sorption generally includessorbing a quantity of a first compound via a MOF, and subsequentlysorbing a quantity of a second compound via the same MOF while at leasta portion of the quantity of the first compound remains sorbed.Simultaneous sorption generally includes contacting a plurality ofcompounds with an MOF, and sorbing a quantity of each of the twocompounds with the MOF.

Sorbing 104 can be reversible. FIG. 1D illustrates a block flow diagramof a method 110 for removing CO₂ from a fluid via a SIFSIX-n-M MOF.Method 110 includes contacting 102 one or more SIFSIX-n-M MOFcompositions with a fluid, sorbing 104 CO₂ from the fluid with the oneor more SIFSIX-n-M MOF compositions, and desorbing 106 CO₂ from the oneor more SIFSIX-n-M MOF compositions. Method 110 can constitute onesorbing cycle. SIFSIX-n-M MOFs can perform a plurality of sorbing cyclesand maintain structural integrity. Optimal CO₂ energetics of SIFSIX-n-MMOF compositions which are strong, uniform, and relatively low enablereversible sorption.

Methods 100 and 110 can be utilized in the context of breathing systems.Specifically, sorbing 104 can be utilized within a breathing system.Efficient removal of CO₂ at low concentrations is vital for the properoperation of breathing systems in confined spaces such as submarines andaerospace shuttles. During long-term space flight and submarinemissions, CO₂ must be removed from the air and recycled because resupplyopportunities are scarce. An average crew member requires approximately0.84 kg of oxygen and emits approximately 1 kg of carbon dioxide. Thus,the ability to continuously purify the exhaled air (with a maximum CO₂concentration of 2-5%) will lead to an optimal recycling andconsiderable reduction in fresh air supply in remote confined spaces.The problem of the existing technologies is the capture capacity/daywhich is low due to mainly to the long temperature swing adsorptioncycling mode (TSAM). The TSAM is mainly determined by the way theadsorbent is cleaned. In the case of low CO₂ concentration removal,chemical (amine supported) adsorbents are preferred with a Heat ofadsorption of 70-100 kJ/mol,—a parameter indicative of the energyrequired to clean the material after each adsorption cycle. ImplementingMOF-based physical adsorption (such as SIFISIX-Cu-3) in a process suchas VTSA or VSA (with mild vacuum) will increase the CO₂ removalcapacity/day and decrease the energy penalty needed for regeneration.

Methods 100 and 110 can be utilized in the context of anaesthesia.Specifically, sorbing 104 can be utilized within an anesthetic system.The use of anaesthesia machines is a growing clinical trend worldwide,driven by the need to reduce costs and improve patient care via the useof efficient CO₂ sorbents. CO₂ removal features in anaesthesia machinesare particularly important in semi-closed or closed rebreathing systems,as the rebreathing fraction is at least 50% of the exhaled gas volume.Exhaled gas volume is directed back to the patient after proper CO₂removal. Sodalime is currently the sorbent of choice in mostcommercially available anaesthesia machines. This sorbent exhibits ahigh CO₂ removal efficiency from exhaled air, with an average continuousoperation of about 24 hours using a pre-packed commercial cartridge.Nevertheless, a major drawback of this technology is that one sodalimecartridge can only be used for a single cycle and is non-recyclable,generating therefore undesirable waste that should be disposed properly.

In case of anaesthesia machines, the use of recyclable SIFSIX-n-M MOFsallow the same regeneratable cartridge much longer durations than 24hours. A single SIFSIX-n-M MOF cartridge can be in operation while twoof the same are in regeneration, for example. Assuming the CO₂ uptake ofMOF is 10 times lower than sodalime but its life time is 10000 higherthan the commercial adsorbents, this can lead to an increase in theoverall capacity by 1000 times. SIFSIX-n-M MOF cartridges can be replaceexisting cartridges, such as sodalime cartridges, to without any majorchanges in the existing anaesthesia machines. The main change in case ofthe recyclable MOF can be the addition of small devices (desorber) forre-activation of the MOF cartridges or in-situ continuoustemperature-pressure adsorption system.

Methods 100 and 110 can be utilized in the context of alkaline fuelcells (AFCs). Specifically, sorbing 104 can be utilized within an AFC.AFCs require a CO₂-free feedstock of oxygen and hydrogenfuel, as eventrace amounts of CO₂ (i.e. 300 ppm) can degrade AFC electrolytes.through progressive carbonation. During typical AFC operation, air istransmitted through the gas diffusion layer of the cathode to thecatalyst layer which can include a KOH solution. Any CO₂ present in theair can react with the KOH to form K₂CO₃ in the catalyst layer, therebyreducing fuel oxidation and oxygen reduction kinetics and AFC poweroutput, inducing precipitation of carbonate salts in porous AFCelectrodes, and reducing AFC electrolyte conductivity. SIFSIX-n-M MOFscan capably remove trace amounts of CO₂ from any air contacting AFCcatalyst layers. The ability of SIFSIX-n-M MOFs to regenerate (i.e.,desorb CO₂ while retaining structural integrity) after CO₂ sorptionlends further benefits to fuel cell applications such as militaryvehicles, which can lack access to fresh CO₂ sorbents while in combat.

Example 1: Synthesis of SIFSIX-3-Cu MOF

A methanol solution (5.0 mL) of pyrazine (pyz, 0.30 g, 3.0 mmol) waslayered in a glass tube onto a methanol solution (5.0 mL) of CuSiF₆.xH₂O(0.325 g, 0.6 mmol). Upon layering, a fast formation of light violetpowder was observed, and the powder was left for 24 hours in the mothersolution. The SIFSIX-3-Cu powder was then collected and washedextensively with methanol then dried under vacuum. The thermalgravimetric analysis (TGA) of the SIFSIX-3-Cu showed a weight loss ofabout 10% for the dried sample in the range of 50-150° C. attributed toguest molecules. From PXRD measurements, the cell parameters,a=b=6.919(1) Å, c=7.906(1) Å, were refined by a whole powder pattern fitusing the Le Bail method, implemented in FULLPROF software. The finalRietveld refinement yielded: R_(Bragg)=0.051 and R_(Factor)=0.056.

FIG. 2A illustrates CO₂ adsorption isotherms at variable temperaturesfor the SIFSIX-3-Cu MOF. The Cu analogue shows the same promisingadsorption properties as SIFSIX-3-Zn analogues. Moreover, the Cuanalogue shows even steeper variable temperature adsorption isotherms atvery low pressures, indicative of relatively stronger CO₂— SIFSIX-3-Cuinteractions. These results emphasize the potential of SIFSIX-3-Cu forCO₂ capture applications.

The SIFSIX-3-Cu MOF exhibited a slightly smaller unit cell as comparedto its Zn analogue (378 vs. 388 Å³). The attributed to the relativelystronger bonding between the Cu(II) and the pyrazine. FIG. 2Billustrates a pore size distribution for the SIFSIX-3-Cu MOF, asdetermined from the CO₂ adsorption isotherms, using a CO₂ at 273 K NLDFTmodel. The relatively sharp pore size distribution (PSD) analysiscentred at 3.5 Å yields a smaller average pore size than the SIFSIX-3-Znanalogue average pore size of 3.84 Å, which is in good agreement withthe determined unit cell sizes. These determinations are supported by arational based on conventional coordination chemistry which suggeststhat replacement of Zn(II) by Cu(II) to form an iso-structuralSIFSIX-3-Cu will potentially induce an additional pore contraction dueto Jahn-Teller distortions of the octahedral coordination geometry ofCu(II), CuN₄F₂. The Cu(II) has an open shell valence electronconfiguration 3d⁹, in contrast to Zn(II) with a close shell 3d¹⁰, andthus will experience a distorted octahedral coordination geometry withpotentially elongated Cu—F (fluorine) bonds and relatively shorter Cu—N(nitrogen) bonds.

Example 2: CO₂ Sorption by Various MOFs

FIG. 3A illustrates CO₂ volumetric uptake for SIFSIX-3-Cu t 298 Kcompared to SIFSIX-3-Zn, SIFSIX-2-Cu—I and Mg-MOF-74. FIG. 3Billustrates isosteric heats of adsorption at low coverage forSIFSIX-3-Cu, SIFSIX-3-Zn and SIFSIX-2-Cu—I. Upon the substitution of Znby Cu, the Q_(st) of CO₂ adsorption in the contracted structureincreased by 20%, from 45 to 54 kJ mol⁻¹ (FIG. 3b ), in perfectagreement with the relatively steeper CO₂ adsorption isotherms in thecase of the Cu analogue at very low pressure. This increase is mainlyattributed to the small unit cell and the contracted pore size of the Cuanalogue which in turn tend to increase the electron density surroundingthe adsorbed CO₂ molecules. The Q_(st) of CO₂ adsorption is an intrinsicproperty that dictates the affinity of the pore surface toward CO₂,which in turn plays a major role in determining the adsorptionselectivity and the necessary energy to release CO₂ during theregeneration step.

Although the Q_(st) for CO₂ was slightly above the typical range offully reversible CO₂ adsorption (30-50 kJ mol⁻¹),²² SIFSIX-3-Cu wasfully and quickly evacuated at 323 K in vacuum (or under N₂ flowenvironment). As in case of SIFSIX-3-Zn and SIFSIX-2-Cu-i, the Q_(st)for CO₂ adsorption was mostly constant up to relatively high CO₂loadings indicating homogenous binding sites over the full range of CO₂loading. The further increase of CO₂ Qst for SIFSIX-3-Cu at the averageloading of 1.5 mmol·g⁻¹ can be explained by the spark of the CO₂—CO₂interactions or possible experimental errors close to the saturation(plateau) of adsorption isotherm.

Example 3A: Trace CO₂ Uptake of Various MOFs

The steep CO₂ adsorption isotherms over a wide range of temperaturesexhibited by SIFSIX-n-M MOFs suggest potential for the same for traceCO₂ adsorption applications (e.g., diluted streams in vacuum or inmixtures containing a large fraction of N₂ up to 95%). In order tohighlight the concealed potential of these MOFs for low CO₂concentration applications (i.e. involving CO₂ concentration below 5%,below 50 mbar CO₂ partial pressure, such as anaesthesia machines andpre-purification before air separation and air capture), single gas CO₂adsorption properties were evaluated for SIFSIX-2-Cu-i and SIFSIX-3-Zn.Table 1 summarizes the CO₂ adsorption uptake at variable low CO₂concentration (partial pressures) for SIFSIX compounds as compared toMg-MOF-74 and amine supported materials (including MOFs), relevant todifferent traces CO₂ removal applications. The SIFSIX-3-Cu MOF alsoshowed even higher CO₂ uptake at 400 ppm and 328 K than thecorresponding uptake at 323 K for amine functionalized Mg-dobpdc-mmen(data not included).

TABLE 1 CO₂ adsorption uptake at various traces CO₂ concentration and at298 K in comparison to the most promising MOFs and other various aminesupported materials. uptake at Uptake at Uptake at CO₂ Qst 400 ppm 5000ppm 10000 ppm (kJ · Adsorbent (0.4 mbar) (5 mbar) (10 mbar) mol⁻¹)SIFSIX-2- 0.0684^(c)/0.2^(d)  0.097^(c)/2.7^(d)   0.19^(c)/5.32^(a)  32Cu-i SIFSIX-3-  0.13^(c)/5.6^(d) 1.12^(c)/39.26^(d) 1.53^(c)/53.97^(d)45 Zn SIFSIX-3-  1.24^(c)/43.9^(d) 2.26^(c)/79.8^(d)  2.34^(c)/82.5^(d) 54 Cu Mg-MOF- 0.088^(c)/1.8^(d) 0.7^(c)/14.3^(d) 1.27^(c)/25.86^(d) 4774 Mg-MOF-  1.5^(c) ND ND ND 74-ED^(a,g) Mg-dobpdc- 2^(c) 2.5^(c) 2.75^(c) 70 mmen^(b,g) TRI-PE- 1^(c)  1.45^(c) 1.6^(c) 92 MCM-41^(f,g)HAS^(f,g)  1.7^(c) ND ND ND ^(a)Ethylenediamine functionalized;^(b)N,N-dimethylethylenediamine functionalized; ^(c)mmol · g⁻¹; ^(d)cm³(STP)/cm³; [e] at 328 K; ^(f)Amine supported silica; ND: non determined.^(g)Chemical adsorbent

The contraction of the pore size from 5.15 Å (for SIFSIX-2-Cu-i) to 3.8Å (for SIFSIX-3-Zn) has prompted a drastic increase in CO₂ uptake andconsequently a recorded highest CO₂ uptake ever reported for a given MOFin the range under 5% CO₂. Specifically, SIFSIX-3-Zn showed an order ofmagnitude higher volumetric CO₂ uptake (55 cm³ (STP)/cm³) than othermaterials such as Mg-MOF-74, (28 cm³ (STP)/cm³) at 10 mbar (1% CO₂),while UTSA-16, exhibits much lower CO₂ uptake similar to SIFSIX-2-Cu-i.

In the context low concentration applications (400 ppm-5%), the exhibitssteep adsorption isotherms at very low CO₂ concentration of theSIFSIX-3-Cu analogue, as shown in FIG. 3A, translate into the highestuptake ever reported for MOFs without unsaturated metal centers (UMCs)or exposed amino functionality at low CO₂ pressures below 38 torr (0.05bar). This can be even more appealing owing to its fully physicaladsorption nature where complete and fast desorption of CO₂ wasestablished under vacuum at only 323 K. At 7.6 torr (0.01 bar)SIFSIX-3-Cu uptakes 82.6 cm³(STP)·cm⁻³ vs. 55 and 28 cm³(STP)·cm⁻³ forSIFSIX-3-Zn and Mg-MOF-74, respectively. The gravimetric uptake ofSIFSIX-3-Cu at 400 ppm and 298 K (1.24 mmol·g⁻¹) is ca. 10 and 15 timeshigher than the corresponding uptakes for SIFSIX-3-Zn (0.13 mmol·g⁻¹)and Mg-MOF-74 (0.08 mmol·g⁻¹) and even higher than the observed uptakesfor most amine-supported silica materials (with optimal compromise ofamine loading and kinetics) at 298 K (for example TRI-PE-MCM-4 (1mmol·g⁻¹)^(10, 22)).

Example 3B: Column Breakthrough Tests of SIFSIX MOFs

The CO₂ selectivity exhibited by SIFSIX-3-Zn and SIFSIX-3-Cu MOFs wasinvestigated experimentally at trace CO₂ concentrations using columnbreakthrough tests for binary CO₂/N₂: 1000 ppm/99.9% mixtures at 298 Kin dry conditions, as well as in humid conditions. FIG. 4A illustrates acolumn breakthrough test of CO₂/N₂:1000 ppm/99.9% for SIFSIX-3-Cu andSIFSIX-3-Zn MOFs in dry conditions. FIG. 4B illustrates a columnbreakthrough test of CO₂/N₂:1000 ppm/99.9% for SIFSIX-3-Cu in dryconditions as well as at 74% relative humidity (RH). In dry conditions,the first CO₂ signal downstream the column was observed only after ca.798 and ca. 1922 min·g⁻¹ for SIFSIX-3-Zn and SIFSIX-3-Cu, respectivelyafter starting continuous CO₂/N₂ gas mixture flux (5 cm³·min⁻¹), whileN₂ breakthrough occurred immediately within a few seconds. Accordingly,at 1000 ppm CO₂ and breakthrough time, SIFSIX-3-Cu shows higherselectivity (ca. 10500) than SIFSIX-3-Zn (7259). It should be noted thatcalculated and measured selectivity exceeding 1000-2000 are oftensubject to uncertainties associated with measurement of the gas uptakeof weakly adsorbed gases (N₂) in the mixture, thus the reportedselectivity is highly qualitative and aimed mainly for relativecomparison of the studied compounds in this work. The steeper CO₂ signalafter breakthrough for SIFSIX-3-Cu as compared to the Zn analogue is adirect indication of the steeper CO₂ adsorption for the Cu analogue asshown in FIG. 3A.

The CO₂ removal selectivity at 1000 ppm CO₂ for SIFSIX-3-Cu MOFs was notaffected by the presence of humidity as shown from the columnbreakthrough tests performed on both compounds at the relative humidity(RH) of 74%. This unprecedented finding was also valid in case ofSIFSIX-3-Zn for the removal of low and higher CO₂ concentration.Finally, as was demonstrated for SIFSIX-3-Zn, SIFSIX-3-Cu is arecyclable and moisture stable MOFs.

Example 4: CO₂ Uptake Kinetic Study for SIFSIX-3-Cu MOFs

Kinetic studies of CO₂ and CO₂/N₂:10/90 adsorption on SIFSIX-3-Cu werecarried out using the Rubotherm gravimetric apparatus operating indynamic regime. Initially, the SIFSIX-3-Cu MOF was properly evacuated at323 K in vacuum. In order to achieve an immediate constancy of pressure(0.5 bar) during kinetics tests and avoid the often noisy uptake duringthe rapid introduction of the studied gas, an initial baseline wasset-up using helium gas at 0.5 bar for single gases and 1 bar formixture, then the studied single gas or mixture is flushed with a flowof 300 ml/min to avoid any dependence of the kinetics on the mass flowcontroller.

The mechanistic behind the unprecedented selective CO₂ adsorptioninvolving the unique synergetic effect of thermodynamics and kineticswas confirmed by the competitive kinetics of CO₂/N₂: 10/90 gas mixtureadsorption. As anticipated, the uptake at equal times for variable CO₂compositions mixtures follows the behaviour of pure CO₂. Further, thetotal uptake of the CO₂ containing gas mixtures at equilibrium overlayperfectly with the equilibrium uptake for pure CO₂. These findings showthat similarly to SIFSIX-3-Zn, when CO₂ containing mixtures are incontact with SIFSIX-3-Cu, CO₂ adsorbs stronger and faster than N₂, and,by analogy, also O₂, CH₄ and H₂. Adsorbed CO₂ thus occupies allavailable physical space and adsorption sites and subsequently excludeother gases, a desirable feature in many CO₂ separation and purificationapplications.

Example 5: Devices for CO₂ Removal

FIG. 5 illustrates a schematic view of a device 500 for removing CO₂from a gas, comprising a gas flow inlet 1, a housing for containing oneor more SIFSIX-n-M MOF compositions 2 for sorbing CO₂, and a gas flowoutlet 3. Sorbing can comprise absorbing, adsorbing, or a combination ofabsorbing and adsorbing. Gas can pass through gas flow inlet 1 and outgas flow outlet 3. Gas can include a mixture of gases having a CO₂concentration of less than 5%.

What is claimed is:
 1. A device for removing CO₂ from a gas, comprising: a gas flow inlet; a housing including a SIFSIX-3-Cu metal-organic framework (MOF) composition for sorbing and/or desorbing CO₂; and a gas flow outlet.
 2. The device of claim 1, wherein the housing further includes one or more SIFSIX-n-M MOF compositions, wherein n is at least 2 and M is a metal other than Cu.
 3. The device of claim 2, wherein the metal is selected from Zn, Co, Mn, Mo, Cr, Fe, Ca, Ba, Cs, Pb, Pt, Pd, Ru, Rh, and Cd.
 4. The device of claim 2, wherein n is
 3. 5. The device of claim 2, wherein at least one of the SIFSIX-n-M MOF compositions includes a bidentate organic ligand.
 6. The device of claim 2, where at least one of the SIFSIX-n-M MOF compositions includes a bidentate N-donor organic ligand.
 7. The device of claim 6, wherein the bidentate N-donor organic ligand includes a cyclic moiety.
 8. The device of claim 2, wherein at least one of the SIFSIX-n-M MOF compositions includes a pyrazine or dipryidilacetylene ligand.
 9. The device of claim 2, wherein at least one of the SIFSIX-n-M MOF compositions is a SIFSIX-3-Zn MOF. 